Traditionally, commercial airplanes were constructed with fuselage structures composed mainly of aluminum. These airplanes were often designed such that multiple tubes carrying hydraulic fluid were required to penetrate the fuselage at different points. Often, fittings were included at these penetration points to facilitate passage of the tubes through the fuselage and to provide for proper attachment of the hydraulic tubes to the fuselage structure in a way that restrained tube movement and prevented damage to both the tube and structure. In addition, in cases where the fuselage was pressurized, these fittings were designed to minimize the leakage through the penetration hole of the tube. Finally, the fitting served the additional purpose of facilitating aircraft assembly and servicing by acting as a connector for tubes being connected from outside the fuselage, thereby allowing external portions of the tubes to be connected and disconnected without significant disruption of the fuselage structure.
In more recent times, commercial airplanes in increasing numbers are being designed and constructed with composite fuselage and wing structures, meaning these structures incorporate components of metal and components composed of other materials. Some of the most common non-metallic materials to be used in aircraft construction are polymer-based materials. These materials are relatively inexpensive and lightweight while at the same time providing sufficient strength for many applications, and as such, designers are making significant use of those materials.
While polymer-based materials offer several advantages in aircraft construction, they also create several challenges. Presently, titanium hydraulic system tubes are used to transport hydraulic fluid through polymer-based thermoset composite fuselages. At certain instances, the fluid in these tubes may have operating temperatures high as 275° F. Using standard fittings designed for aluminum fuselages, the penetration of these tubes through the fuselage skin can lead to localized areas near the penetration hole with temperatures elevated to a point where thermoset composite material aging and degradation is a concern. As such, a need in the art for a new fitting that allows tube penetration through a thermoset composite fuselage while avoiding excessive localized heating.
Another challenge presented by the use of polymer-based materials in aircraft construction is the unwanted transmission of vibrational energy. Hydraulic system components, such as motors, pumps, and actuators, tend to vibrate as they operate. These vibrations may be transmitted along the hydraulic tubes leading to and away from the hydraulic components. When the hydraulic tubes pass through and come in contact with the fuselage, some of the vibrational energy may be transmitted to the fuselage. Additionally, because the hydraulic tubes are carrying a fluid, some of the energy associated with the movement of the fluid may be also transmitted to the fuselage. This vibrational energy may cause the fuselage to vibrate and produce noise in the fuselage. In the past, the damping capacity of the aluminum plane structures was sufficient to maintain the vibration-induced fuselage noise at an acceptable level. However, the capacity of composite fuselage structures to dampen mechanical vibrations is less than that of the aluminum structures, mainly due to the characteristics of the materials used in the composite fuselages. Hence, there is a need in the art for a new fitting that will dampen vibrations being transmitted from hydraulic tubes to composite fuselages through which the tubes are penetrating.